Electronic scanning structures may scan in two orthogonal directions; cylindrical or conical structures assuring scanning in the plane containing the axis of the structure.
The present invention pertains to an electronic scanning method starting with a microwave beam emitted by a flat antenna, and to the applications of this method to the construction of structures permitting electronic scanning by a focused beam without dispersion caused by variations of the emitted frequency.
We know that the microwaves employed in radar systems employing either mechanical or electronic scanning processes may generate plane waves. These plane waves can be produced for example by an optical focusing system of non-directional spherical waves emitted by horns or by simple dipoles.
We well know the inconveniences of these systems, such as parabolic reflector or passive focusing lens, due to their bulkiness, their lack of manipulation, or difficulty in adjusting. There also exist systems emitting plane waves. These are flat antennas which can be made up of slotted wave guides, dipole stripline circuits or other forms of multifunction integrated circuits.
These flat antennas are made up of a juxtaposition of single radiating elements in rows and columns; each of these single elements is not fed individually but each line of elements is fed at one end in order to reduce the number of feeders.
These types of flat antennas are interesting in that they produce a focused plane wave without the addition of supplementary equipment. They possess however a well known but bothersome fault in that since these single elements which make up the flat antenna are fed in series at one end and not individually, the plane wave radiated by the flat antenna is not exactly parallel to the plane of the single elements; furthermore the direction angle this wave takes with respect to the perpendicular of the array plane varies with the frequency of the wave.
Certain authors have contended that one could obtain a scanning of the beam in a plane parallel to the slots of a slotted wave guide and perpendicular to the antenna array by varying the frequency of the radiated wave. What this describes is not in fact a true scanning but a variation in the pointing angle of the radiated beam in a very narrow and reduced region.
This variation is more a dispersion than a scanning since the possibility of scanning by varying the radiated frequency about a nominal frequency is very limited, because a flat antenna focuses its energy about a determined direction close to the direction perpendicular to the antenna plane only for those frequencies very close to the optimum or design frequency of the antenna.
It is possible however to cause scanning starting with the microwaves radiated by a flat antenna. Two solutions are known both of which have important disadvantages.
One solution consists in mechanically orienting a flat antenna. The beam radiated by a flat antenna which is subject to the above-mentioned dispersion is not always pointed in a direction perpendicular to the plane of the antenna and hence describes a cone. Accordingly, for scanning in a plane to be complete, a movement of the antenna along two perpendicular axes is indispensible.
The other solution consists of achieving an electronic scanning by placing phase shifters in series with each feed of the wave guides which make up the flat antenna. This solution may appear to cause a scanning in the direction perpendicular to the series of wave guides. This solution, however, is not satisfactory since the resulting scanning is in fact in a direction perpendicular to the dispersion caused by the variations in frequency and the effect of this dispersion is not compensated for.
The actual tendency for microwave scanning is to use a large enough frequency band of plus or minus ten percent of the nominal frequency centered about the nominal frequency. Accordingly, the effect of dispersion can not be ignored.
A flat antenna, even supplemented by a substantial aggregate of phase shifters, is difficult to control electronically and to use for high radiated power and will always have the major inconvenience of conical movement due to the beam dispersion for frequency variations during the scanning process.
The present invention has for its main objective the elimination of these problems and of furnishing methods and apparatus for electronic scanning starting with an electromagnetic wave radiated from a flat antenna in which there exists no dispersion in the pointing angle of the resultant beam when the radiated frequency varies about the nominal or design frequency of the flat antenna.
Additional objects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.